Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention discloses a battery diaphragm and a preparation method thereof, and a secondary battery, and belongs to the field of secondary batteries, the battery diaphragm comprises a base membrane and a nanofiber layer stacked on at least one side surface of the base membrane, the nanofiber layer is prepared from the following components in percentage by mass: 8 to 15 parts of nanocellulose, 9 to 20 parts of second inorganic particles, 0.05 to 0.1 part of second dispersing agent, 0.3 to 0.5 part of second thickening agent, 1 to 2 parts of second binding agent and 0.05 to 0.1 part of second wetting agent. Wherein the nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals and bacterial nanocellulose; the inorganic particles comprise at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles and zirconium oxide particles. The battery diaphragm has good heat resistance and liquid absorption and retention properties.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a battery separator and a preparation method thereof, and a secondary battery. Background Art

[0002] The separator in a secondary battery mainly separates the positive and negative electrodes and allows ions to pass freely. Currently, the separator is usually made of polyethylene-based film, which has a low melting point (approximately 140°C-145°C), making it less heat-resistant.

[0003] In order to enhance the heat resistance of the diaphragm, a ceramic layer is usually coated on the surface of the polyethylene-based film. However, the battery diaphragm with a ceramic layer will still undergo large thermal shrinkage deformation when the temperature is greater than 150°C, with a thermal shrinkage rate of up to 10%, which can easily cause thermal runaway of the battery. Summary of the Invention

[0004] In view of this, the present invention provides a battery separator and its preparation method, and a secondary battery, which can solve the technical problem of poor heat resistance of battery separators in related technologies. Specifically, it includes the following technical solutions:

[0005] In one aspect, an embodiment of the present invention provides a battery separator, comprising: a base film and a nanofiber layer laminated on at least one surface of the base film, wherein the nanofiber layer comprises the following components in percentage by weight: 8-15 parts of nanocellulose, 9-20 parts of second inorganic particles, 0.05-0.1 parts of a second dispersant, 0.3-0.5 parts of a second thickener, 1-2 parts of a second binder, and 0.05-0.1 parts of a second wetting agent;

[0006] Wherein, the nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals, and bacterial nanocellulose;

[0007] The inorganic particles include at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles.

[0008] In some possible implementations, the nanocellulose has a diameter of 5 nm to 50 nm and a length of 1 μm to 10 μm.

[0009] In some possible implementations, the specific surface area of ​​the inorganic particles is 8 m 2 / g-16m 2 / g and the particle size distribution of the inorganic particles is as follows: D10≥0.1μm, D50 is 0.3-0.6μm; D90≤1.8μm; D99≤3.0μm.

[0010] In some possible implementations, the dispersant is selected from at least one of polyvinyl alcohol, sodium dodecylbenzenesulfonate, glycerol, and polymethyl acrylate.

[0011] In some possible implementations, the thickener is sodium hydroxymethyl cellulose.

[0012] In some possible implementations, the binder is selected from at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, silicone-modified polyacrylate, polyurethane-modified polyacrylate, and methacryloyl epoxy ester.

[0013] In some possible implementations, the wetting agent is selected from at least one of a polyether wetting agent, a silicone and polyether mixture wetting agent, and an alcohol alkoxylate wetting agent.

[0014] In some possible implementations, the thickness of the nanofiber layer is 5 μm-10 μm, and the air permeability increment of the nanofiber layer is 10 s / 100 ml-200 s / 100 ml.

[0015] In some possible implementations, the base film is a polyethylene base film or a polypropylene base film.

[0016] In another aspect, a method for preparing a battery separator is provided. The battery separator is as described above, and the method for preparing the battery separator comprises:

[0017] The nanofiber slurry is coated on at least one side of the base film, and after drying, the battery separator is obtained;

[0018] The nanofiber slurry includes a composition for forming a nanofiber layer and deionized water.

[0019] On the other hand, a secondary battery is provided, comprising: a shell, an electrolyte contained inside the shell, a negative electrode plate, a positive electrode plate and a battery separator, wherein the negative electrode plate and the positive electrode plate are separated by the battery separator, and the battery separator is as described above.

[0020] The beneficial effects of the technical solution provided by the embodiment of the present invention include at least:

[0021] The battery separator provided by an embodiment of the present invention has a nanofiber layer provided on the surface of its base membrane. The nanofiber layer contains nanocellulose and inorganic particles. The nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals, and bacterial nanocellulose, and each of the above nanocelluloses has excellent heat resistance. The inorganic particles include at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles, and each of the above inorganic particles also has excellent heat resistance. The nanocellulose and inorganic particles work synergistically in a certain ratio, which can significantly improve the heat resistance of the battery separator and optimize the battery separator's membrane rupture temperature and thermal shrinkage rate at high temperatures. At the same time, the surface of the nanocellulose contains abundant hydroxyl functional groups, which have a strong affinity with the electrolyte, making the battery separator also have strong liquid absorption and liquid retention. In addition, by doping the nanofiber layer with a certain amount of dispersant, thickener, wetting agent, and binder, it is not only beneficial to enhance the bonding strength between the nanofiber layer and the base membrane, but also to improve the uniformity of the distribution of the nanocellulose and inorganic particles in the nanofiber layer, thereby optimizing its heat resistance and liquid retention. DETAILED DESCRIPTION

[0022] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0023] On the one hand, an embodiment of the present invention provides a battery separator, which includes: a base film and a nanofiber layer laminated on at least one surface of the base film, wherein the nanofiber layer includes the following components in percentage by weight: 8-15 parts nanocellulose, 9-20 parts second inorganic particles, 0.05-0.1 parts second dispersant, 0.3-0.5 parts second thickener, 1-2 parts second binder, and 0.05-0.1 parts second wetting agent. The nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals, and bacterial nanocellulose. The inorganic particles include at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles.

[0024] The battery separator provided by the present invention comprises a nanofiber layer containing multiple strands of nanocellulose and a predominant number of inorganic particles. The nanocellulose serves as the filler, while the inorganic particles form the backbone. The two are stably bonded together by a binder, forming the bulk of the nanofiber layer. Furthermore, the synergistic effects of a dispersant, thickener, and wetting agent ensure uniform distribution of the nanocellulose and inorganic particles within the nanofiber layer, further optimizing its overall performance.

[0025] One example is that the nanofiber layer can be arranged on one side of the base membrane, and another example is that the nanofiber layer can be arranged on both sides of the base membrane. The more nanofiber layers there are, the larger the average thickness of the battery separator will be and the better the heat resistance will be. The number of nanofiber layers can be selected according to the average thickness requirements of the battery separator in different thermal management scenarios.

[0026] The battery separator provided by an embodiment of the present invention has a nanofiber layer provided on the surface of its base membrane. The nanofiber layer contains nanocellulose and inorganic particles. The nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals, and bacterial nanocellulose, and each of the above nanocelluloses has excellent heat resistance. The inorganic particles include at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles, and each of the above inorganic particles also has excellent heat resistance. The nanocellulose and inorganic particles work synergistically in a certain ratio, which can significantly improve the heat resistance of the battery separator and optimize the battery separator's membrane rupture temperature and thermal shrinkage rate at high temperatures. At the same time, the surface of the nanocellulose contains abundant hydroxyl functional groups, which have a strong affinity with the electrolyte, making the battery separator also have strong liquid absorption and liquid retention. In addition, by doping the nanofiber layer with a certain amount of dispersant, thickener, wetting agent, and binder, it is not only beneficial to enhance the bonding strength between the nanofiber layer and the base membrane, but also to improve the uniformity of the distribution of the nanocellulose and inorganic particles in the nanofiber layer, thereby optimizing its heat resistance and liquid retention.

[0027] The nanocellulose is selected from at least one of cellulose microfibrils (CNF), nanocellulose crystals (CNC), and bacterial nanocellulose (BNC).

[0028] Cellulose microfibrils are mainly extracted from plant fibers (such as wood and crop straw). The hydrogen bonds between cellulose fibers are destroyed through mechanical treatment (such as high-pressure homogenization and ball milling) or chemical pretreatment (such as TEMPO oxidation) to peel off the cellulose microfibrils. In addition to having strong heat resistance, they also have high strength, modulus and large specific surface area.

[0029] Nanocellulose crystals are obtained by hydrolyzing plant cellulose with strong acids (such as sulfuric acid), selectively removing the amorphous regions, and retaining rigid nanoparticles with high crystallinity (70-95%). In addition to having strong heat resistance, they also have high strength, hardness, good solubility and processability.

[0030] Bacterial nanocellulose is synthesized by fermentation of specific bacteria (such as Acetobacter xylinum) in a sugar-containing culture medium. In addition to its strong heat resistance, it also has high water retention and air permeability.

[0031] Nanocellulose can be made from cellulose microfibrils, nanocellulose crystals, or bacterial nanocellulose, or from two or all of them. The appropriate nanocellulose and its content can be selected based on actual needs.

[0032] In some examples, the diameter of the nanocellulose is 5 nm to 50 nm, and the length is 1 μm to 10 μm. For example, the diameter of the nanocellulose can be any of the following values ​​or an interval consisting of two of them: 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, etc., and the length of the nanocellulose can be any of the following values ​​or an interval consisting of two of them: 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0033] By limiting the size of the nanocellulose as above, so that it is at the nanoscale diameter, micron-scale length and large specific surface area, the nanofiber layer has at least the following advantages: enhancing the mechanical properties of the nanofiber layer, increasing the porosity and air permeability of the nanofiber layer, improving the lyophilicity and wettability of the nanofiber layer, enhancing the interfacial interaction between the nanocellulose and other components, and enabling the components to work better synergistically.

[0034] For example, the mass fraction of nanocellulose in the nanofiber layer can be any of the following values ​​or an interval consisting of two of the following values: 8 parts, 8.5 parts, 9 parts, 9.5 parts, 10 parts, 10.5 parts, 11 parts, 11.5 parts, 12 parts, 12.5 parts, 13 parts, 13.5 parts, 14 parts, 14.5 parts, 15 parts, etc.

[0035] The inorganic particles in the nanofiber layer include at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles. For example, the inorganic particles may be boehmite particles. These inorganic particles all have high hardness and strength, as well as high heat resistance, which is beneficial for improving the strength and heat resistance of the battery separator.

[0036] The mass fraction of inorganic particles in the nanofiber layer can be any of the following values ​​or an interval consisting of two of the values: 9 parts, 10 parts, 11 parts, 12 parts, 13 parts, 14 parts, 15 parts, 16 parts, 17 parts, 18 parts, 19 parts, 20 parts, etc.

[0037] In some examples, the inorganic particles have a specific surface area of ​​8 m 2 / g-16m 2 / g and the particle size distribution of the inorganic particles is as follows: D10 ≥ 0.1 μm, D50 is 0.3-0.6 μm; D90 ≤ 1.8 μm; D99 ≤ 3.0 μm.

[0038] The particle sizes D10, D50, D90, and D99 refer to the particle sizes corresponding to the cumulative distributions reaching 10%, 50%, 90%, and 99% respectively in the particle size distribution.

[0039] Inorganic particles have a high specific surface area. This, on the one hand, strengthens the bonding between the inorganic particles and other components such as nanocellulose and binders, improving the structural stability and integrity of the nanofiber layer. On the other hand, it helps increase the contact area between the inorganic particles and the electrolyte, thereby improving the separator's ability to adsorb and retain the electrolyte, making the electrolyte more evenly distributed within the separator, which helps improve the battery's ionic conductivity, and thus enhances the battery's charge and discharge efficiency and cycle performance.

[0040] For the particle size distribution of inorganic particles, D10 ≥ 0.1 μm ensures the presence of a certain number of smaller inorganic particles. These small particles can fill the gaps between larger particles, making the pore structure of the separator more uniform and dense. D50 is 0.3-0.6 μm, indicating that the average particle size of the inorganic particles is moderate, which helps to form pores of appropriate size, which is conducive to the rapid transmission of lithium ions while preventing direct conduction of electrons, thereby improving the ion selective permeability of the separator. D90 ≤ 1.8 μm and D99 ≤ 3.0 μm indicate that the proportion of larger particles is relatively small, avoiding the presence of particles with excessively large sizes that lead to a decrease in the mechanical properties of the separator, such as reduced puncture resistance and poor flexibility. At the same time, this narrow particle size distribution range helps to reduce the agglomeration of inorganic particles. Particles with a uniform particle size distribution are easier to disperse evenly in the separator, avoiding local performance differences caused by particle agglomeration, and ensuring the consistency and stability of the separator performance. Moreover, a suitable particle size distribution helps to improve the overall strength and toughness of the separator, enabling it to withstand external forces during battery assembly and use.

[0041] The mass fraction of the dispersant in the nanofiber layer can be any of the following values ​​or a range consisting of two of these values: 0.05 part, 0.06 part, 0.07 part, 0.08 part, 0.095 part, 0.1 part, etc. Some suitable second dispersants can be at least one of polyvinyl alcohol, sodium dodecylbenzenesulfonate, glycerol, and polymethyl acrylate. Each of the above second dispersants can synergistically act with other components in the nanofiber layer to achieve a good dispersion effect.

[0042] The mass fraction of the thickener in the nanofiber layer can be any of the following values ​​or a range consisting of two of these values: 0.3 part, 0.35 part, 0.4 part, 0.45 part, 0.5 part, etc. In some examples, the thickener is sodium hydroxymethyl cellulose. When preparing the nanofiber layer, the sodium hydroxymethyl cellulose can be pre-mixed with water to form a sodium hydroxymethyl cellulose glue with a mass concentration of 0.5%-1%. Subsequently, the sodium hydroxymethyl cellulose glue and other components are dissolved in water to form a slurry.

[0043] The mass content of the binder in the nanofiber layer can be any of the following values ​​or an interval consisting of two of the following values: 1 part, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts, 1.5 parts, 1.6 parts, 1.7 parts, 1.8 parts, 1.9 parts, 2 parts, etc. For example, the binder is an acrylic binder, and the binder can be selected from at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, silicone-modified polyacrylate, polyurethane-modified polyacrylate, and methacryloyl epoxy ester.

[0044] The above-mentioned binder can not only further enhance the bonding force of the nanofiber layer, but also facilitates the formation of an elastic network to inhibit deformation and cracking of the nanofiber layer.

[0045] The wetting agent can reduce the surface tension of the particles and enhance the bonding strength between the nanofiber layer, the base film, and the heat-sensitive layer. In some examples, the wetting agent can be present in any of the following weight percentages, or in a range of two of these weight percentages: 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, etc. The wetting agent can be present in the heat-sensitive layer in any of the following weight percentages, or in a range of two of these weight percentages: 0.05%, 0.06%, 0.07%, 0.08%, 0.095%, 0.1%, etc.

[0046] Exemplarily, the wetting agent is selected from at least one of a polyether wetting agent, a mixture wetting agent of silicone and polyether, and an alcohol alkoxylate wetting agent. Among them, the polyether wetting agent can be, for example, a fatty alcohol polyoxyethylene ether (lauryl alcohol polyoxyethylene ether, etc.), a polyoxyethylene polyoxypropylene block copolymer (poloxamer, etc.), and the mixture wetting agent of silicone and polyether can be, for example, the commercially available product Digo Wet 270, Evonik Wet 290, etc., and alcohol alkoxylate wetting agents can be, for example, isotridecanol polyoxyethylene ether, etc.

[0047] For any of the above-mentioned battery separators, the thickness of the nanofiber layer is 5 μm-10 μm, for example, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc.

[0048] In an embodiment of the present invention, the air permeability increment of the nanofiber layer is 10s / 100ml-200s / 100ml, and can further be 10s / 100ml-100s / 100ml. The air permeability increment of the nanofiber layer refers to the difference between the air permeability value of the battery separator formed by the nanofiber layer + base membrane at room temperature and the air permeability value of the base membrane at room temperature. It can be seen that after the nanofiber layer is coated on the base membrane, it shows a lower air permeability increment and will not deteriorate the air permeability performance of the battery separator.

[0049] For any of the battery separators mentioned above, the base film may be a polyethylene base film or a polypropylene base film.

[0050] Both polyethylene-based films and polypropylene-based films have good chemical stability, high mechanical strength, excellent electrical insulation properties, suitable pore structure and pore size distribution, and are easy to process and shape, giving the battery separator excellent comprehensive performance.

[0051] Furthermore, the base film may meet at least one of the following parameters: a molecular weight of 300,000-3,000,000 g / mol, including but not limited to: 500,000 g / mol-1,500,000 g / mol, 1,500,000 g / mol-3,000,000 g / mol, etc.

[0052] When the molecular weight of the base film is 1.5 million g / mol-3 million g / mol, it has higher mechanical strength (puncture strength is significantly improved), which is beneficial to improving the safety of the battery.

[0053] The base film of the embodiment of the present invention may have a thickness of 1 μm-30 μm, including but not limited to 1 μm-10 μm, 5 μm-15 μm, 10 μm-20 μm, etc. When the base film thickness is within the above range, it is suitable for currently common types of secondary batteries.

[0054] In summary, the battery separator provided by the embodiment of the present invention has a nanofiber layer that uses the synergistic effect of nanocellulose and inorganic particles, and has the advantages of strong density, good structural stability, and not prone to thermal shrinkage when heated. In particular, based on the high melting point (for example, the melting point can be greater than or equal to 250°C) of nanocellulose and inorganic particles, the membrane rupture temperature of the battery separator is ≥250°C, and the thermal shrinkage rate at 200°C is reduced. Moreover, the nanofiber layer contains a large number of hydroxyl groups with good affinity for the electrolyte based on the molecules of nanocellulose, and the nanometer size and high specific surface area of ​​nanocellulose, so that the nanofiber layer has good liquid absorption and retention properties for the electrolyte.

[0055] On the other hand, an embodiment of the present invention further provides a method for preparing a battery separator, wherein the battery separator is as described above, and the method for preparing the battery separator comprises:

[0056] The nanofiber slurry is coated on at least one side of the base film and dried to obtain a battery separator. The nanofiber slurry includes a composition for forming a nanofiber layer and deionized water.

[0057] As described above, the composition for constituting the nanofiber layer includes the following components in percentage by weight: 8-15 parts of nanocellulose, 9-20 parts of second inorganic particles, 0.05-0.1 parts of a second dispersant, 0.3-0.5 parts of a second thickener, 1-2 parts of a second binder, and 0.05-0.1 parts of a second wetting agent.

[0058] In some examples, the solid content of the nanofiber slurry is 18%-38%, and the content of deionized water in the slurry is adaptively determined according to the solid content.

[0059] When preparing the nanofiber layer, the nanofiber slurry is applied to at least one surface of the base film and dried to form the nanofiber layer. During this process, deionized water is removed, that is, the nanofiber layer does not contain deionized water but only includes the components in the composition.

[0060] When the nanofiber slurry is coated on the surface of the base film, the coating process used can be gravure coating, micro gravure coating, transfer coating, spray coating or spin coating, etc. By controlling the coating process parameters, the average thickness of the nanofiber layer can meet the corresponding requirements.

[0061] On the other hand, an embodiment of the present invention provides a secondary battery, which includes: a shell, an electrolyte contained inside the shell, a negative electrode plate, a positive electrode plate and a battery separator, wherein the negative electrode plate and the positive electrode plate are separated by a battery separator, and the battery separator is as described above.

[0062] The secondary battery provided by the embodiment of the present invention has all the advantages of the battery separator mentioned above, which will not be described in detail here. For example, the secondary battery can be a lithium ion battery, a sodium ion battery, etc.

[0063] An embodiment of the present invention also provides an electrical device, which includes the secondary battery involved above. For example, the electrical device can be a portable electronic device (mobile phone, laptop computer, smart wearable device, etc.), new energy transportation equipment (new energy vehicle, etc.), energy storage system, etc.

[0064] Below will be described in more detail exemplary embodiments of the present invention. Although the following describes exemplary embodiments of the present invention, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. In the examples, if specific techniques or conditions are not indicated, they are carried out according to the techniques or conditions described in the literature in this area or according to the product specifications. Reagents or instruments used that do not indicate the manufacturer are conventional products that can be obtained commercially.

[0065] Example 1

[0066] Example 1 provides a battery separator comprising: a polyethylene-based film and nanofiber layers stacked on both sides of the polyethylene-based film, wherein:

[0067] The polyethylene base film is made of polyethylene with a molecular weight of 1 million and a thickness of 7 μm. The thickness of the nanofiber layer is 8 μm. The nanofiber layer includes the following components in percentage by weight: 13 parts of nanocellulose, 20 parts of boehmite particles, 0.08 parts of polyvinyl alcohol, 0.5 parts of sodium hydroxymethyl cellulose, 1.8 parts of polymethyl acrylate, and a wetting agent. Wet 2700.06 parts.

[0068] Among them, nanocellulose is cellulose microfibrils with a diameter of 20nm and a length of 5μm. The specific surface area of ​​boehmite particles is 10m 2 / g and the particle size distribution meets the following requirements: D10 ≥ 0.1 μm, D50 is 0.3-0.6 μm; D90 ≤ 1.8 μm; D99 ≤ 3.0 μm.

[0069] The battery separator is prepared by the following method:

[0070] The composition raw materials corresponding to the nanofiber layer are mixed with deionized water to prepare a nanofiber slurry with a solid content of 20%, and the slurry is coated on both sides of the base film. After drying, the nanofiber layer is obtained, thereby completing the preparation of the battery separator.

[0071] Example 2

[0072] Example 2 provides a battery separator, which differs from Example 1 in that the nanocellulose is selected from nanocellulose crystals with a diameter of 10 nm and a length of 3 μm.

[0073] Example 3

[0074] Example 3 provides a battery separator, which differs from Example 1 in that the nanocellulose is selected from bacterial nanocellulose, has a diameter of 30 nm and a length of 8 μm.

[0075] Example 4

[0076] Example 4 provides a battery separator, which differs from Example 1 in that the formula of the nanofiber layer is different, and the nanofiber layer includes the following components in percentage by weight: 11 parts of cellulose microfibrils, 16 parts of boehmite particles, 0.1 parts of polyvinyl alcohol, 0.3 parts of sodium hydroxymethyl cellulose, 2 parts of polymethyl acrylate, and a wetting agent. Wet 270 0.08 parts.

[0077] Comparative Example 1

[0078] Comparative Example 1 provides a battery separator, which differs from Example 1 in that the nanofiber layer uses nanocellulose instead of inorganic particles. The formula is as follows: nanocellulose 80%, polyvinyl alcohol 2%, sodium hydroxymethyl cellulose 5%, polymethyl acrylate 10%, wetting agent Wet 270 3%.

[0079] Comparative Example 2

[0080] Comparative Example 2 provides a battery separator, which differs from Example 1 in that nanocellulose is not used in the coating. The polyethylene base film is prepared using polyethylene with a molecular weight of 1 million and a thickness of 7 μm. The polyethylene base film is coated with a boehmite coating on both sides, and the thickness of the coating on one side is 2 μm. The formula is as follows: 25 parts of boehmite particles, 0.08 parts of polyvinyl alcohol, 0.5 parts of sodium hydroxymethyl cellulose, 1.8 parts of polymethyl acrylate, and a wetting agent. Wet 270 0.06 parts.

[0081] Test Case

[0082] The following performance tests were performed on the battery separators provided in Examples 1 to 4, and Comparative Examples 1 and 2. The performance test items and test results are shown in Table 1.

[0083] (1) 200℃@1h thermal shrinkage: The test method is carried out in accordance with GB / T36363-2018. Take 5 battery separator samples and measure their dimensions before heating. The samples are sandwiched between 6mm glass plates and placed in a 200℃ oven for 1 hour. The dimensions of the separator samples after heating are measured and their thermal shrinkage is calculated in %. The longitudinal shrinkage (MD) and transverse shrinkage (TD) of the separator samples are measured separately in %.

[0084] (2) Liquid absorption: The battery separator was placed in an electrolyte solution (1 M LiPF6 electrolyte of vinyl carbonate (EC) and dimethyl carbonate (DMC) (v / v = 1:1)) for 1 h. After taking it out, the excess electrolyte solution was quickly washed out with filter paper. The mass change of the separator before and after adsorption was tested, and the liquid absorption percentage was calculated in %.

[0085] (3) Air permeability: The test method for air permeability is carried out in accordance with GB / T458-2008, and the air permeability of the base film and the battery separator (base film + coating) at room temperature is measured respectively. The air permeability increment is the air permeability value of the battery separator minus the air permeability value of the base film. The unit of the air permeability value is s / 100mL.

[0086] Table 1

[0087]

[0088] As shown in Table 1, the battery separators provided in Examples 1-4, due to the improved nanofiber layer formed on the surface of the base membrane, exhibit lower thermal shrinkage at 200°C than those in Comparative Examples 1-2, and their liquid absorption percentage remains above 180%, resulting in both excellent heat resistance and liquid absorption and retention. Furthermore, the battery separators provided in Examples 1-4, with the nanofiber layer formed onto the surface of the base membrane, also exhibit the advantage of lower air permeability gain.

[0089] The battery separator provided in Comparative Example 1 increases the amount of nanocellulose but does not use inorganic particles. Although its liquid absorption is improved, its thermal shrinkage at 200°C is deteriorated, resulting in poor heat resistance.

[0090] The battery separator provided in Comparative Example 2 increases the amount of boehmite particles but does not use nanocellulose. Its liquid absorption and thermal shrinkage at 200°C are deteriorated, showing poor heat resistance and liquid absorption. Moreover, the boehmite coating significantly increases the air permeability increment of the battery separator relative to the nanofiber layer containing nanocellulose.

[0091] The above description is only for the purpose of facilitating those skilled in the art to understand the technical solution of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A battery separator, characterized in that: The battery separator comprises: a base film and a nanofiber layer stacked on at least one side of the base film, wherein the nanofiber layer comprises the following components in parts by weight: 8-15 parts of nanocellulose, 9-20 parts of second inorganic particles, 0.05-0.1 parts of a second dispersant, 0.3-0.5 parts of a second thickener, 1-2 parts of a second binder, and 0.05-0.1 parts of a second wetting agent; Wherein, the nanocellulose is selected from at least one of cellulose microfibrils, nanocellulose crystals, and bacterial nanocellulose; The inorganic particles include at least one of boehmite particles, aluminum oxide particles, magnesium hydroxide particles, barium carbonate particles, magnesium carbonate particles, and zirconium oxide particles.

2. The battery separator according to claim 1, characterized in that The nanocellulose has a diameter of 5 nm to 50 nm and a length of 1 μm to 10 μm.

3. The battery separator according to claim 1, characterized in that The specific surface area of ​​the inorganic particles is 8m 2 / g-16m 2 / g and the particle size distribution of the inorganic particles is as follows: D10≥0.1μm, D50 is 0.3-0.6μm; D90≤1.8μm; D99≤3.0μm.

4. The battery separator according to any one of claims 1 to 3, characterized in that: The dispersant is selected from at least one of polyvinyl alcohol, sodium dodecylbenzene sulfonate, glycerol, and polymethyl acrylate.

5. The battery separator according to any one of claims 1 to 3, characterized in that: The thickener is sodium hydroxymethyl cellulose.

6. The battery separator according to any one of claims 1 to 3, characterized in that: The binder is selected from at least one of polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, silicone-modified polyacrylate, polyurethane-modified polyacrylate, and methacryloyl epoxy ester.

7. The battery separator according to any one of claims 1 to 3, characterized in that: The wetting agent is selected from at least one of a polyether wetting agent, a mixture wetting agent of silicone and polyether, and an alcohol alkoxylate wetting agent.

8. The battery separator according to any one of claims 1 to 7, characterized in that: The thickness of the nanofiber layer is 5 μm-10 μm, and the air permeability increment of the nanofiber layer is 10 s / 100 ml-200 s / 100 ml.

9. The battery separator according to any one of claims 1 to 8, characterized in that: The base film is a polyethylene base film or a polypropylene base film.

10. A method for preparing a battery separator, characterized in that: The battery separator according to any one of claims 1 to 9, wherein the preparation method of the battery separator comprises: The nanofiber slurry is coated on at least one side of the base film, and after drying, the battery separator is obtained; The nanofiber slurry includes a composition for forming a nanofiber layer and deionized water.

11. A secondary battery, characterized in that: The secondary battery comprises: a shell, an electrolyte contained in the shell, a negative electrode sheet, a positive electrode sheet and a battery separator, wherein the negative electrode sheet and the positive electrode sheet are separated by the battery separator, and the battery separator is as described in any one of claims 1 to 9.

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